Klinefelter syndrome: A condition in males who have XXY sex chromosomes, rather than the usual XY. Some also have additional X chromosomes, or more than one Y chromosome. XXY is one of the most common chromosomal abnormalities. It occurs in 1 in 500 male births. The signs of Klinefelter syndrome include small testes, insufficient production of testosterone, and infertility. XXY males are more likely than other males to show breast enlargement, lack of facial and body hair, and a rounded body type, and they are more likely than other males to be overweight and be taller than their fathers and brothers. Klinefelter boys tend to have learning and/or behavioral 'problems.